They are currently doing the AMA and here's what their reasoning is:
1) The reason why Steam is less attractive these days is that discoverability is difficult and challenging, you fall off the front page within a moment. With a 150 games released a week its extremely difficult to know if your game is seen by players
Even with a name of Mechwarrior 5, the game is going to disappear of the frontpage of Steam in a week. With the EGS, the benefit there's only one game a week go to the store. With EGS, there's much longer visibility. PGI has actual sales data on products that went on EGS than with projected sales than on Steam and they are very positive. In fact, far more on EGS than previous iterations on Steam
2) The deal with EGS means higher visibility plus bigger cut. The rate 12% for EGS including the Unreal Engine. With Steam its 35% (30% to Valve, 5% to UE4). 750K units on EGS means 1 millions of sales on Steam. So in a situation where the product is not a big hit, EGS is much safer and PGI gets to the recoup point a lot quicker. I don't expect people to care about PGI's profits, but Russ Bollock has run the company for 20 years so you can bet PGI are going to make the decision that will make the game more successful.
3) Gears of War 5 and Borderlands 3 dropping right on top of MW5 in September meant that it would get buried by those major titles. So they chose to delay and thought about their opportunities and ended up on December. Russ also said this would happen last year if a big game would drop. Russ said that he would change the release date if a big game dropped right on top of MW5 back in 2018 during Mech Con.
4) EGS gives more money and allows for a longer delay. The money allows PGI to hire more people, make localization ready for release, and gives them a much heavier marketing push. The early December release means they can comfortably develop the game without too much pressure (less crunch!). It also means it will be released in German, French and Russian. The marketing budget is much bigger and it's not small. They've also hired more employees like narrative designers, localization, lighting artists.
With all these factors, it was the right decision they made for the studio. The game now has a much better chance to succeed. Russ bases these hopes on actual things on past sales of past Mechwarrior games. The bottom line is shipping a good game. For us we have to stay focused on making a great game and that's all we can focus on. Going to the EGS gives it a better chance for higher sales will lead to other Mechwarrior titles because the game will get better from this delay.
Sales data: Pre-orders for MW5 is 20.000. 700 have asked for a refund today. More will come obviously. (MW:Online got 70.000 back in 2012 but that was during the heyday of Kickstarters)
Sales target: "Our aspirations is that we want to sell a million units. If we hit the quality level needed, we expect the game to sell well."
Steam keys are not allowed for pre-orders any longer because Valve no longer allows this for developers who go EGS exclusive. It changed after the last Metro.
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Morrigan feel free to threadmark this post. It probably is of interest to people interested in the business arrangements between Valve and Epic.